Lithium ion secondary batteries are attracting much attention as not only compact devices for portable electronics but also high-capacity devices for on-vehicle applications and power storage. Therefore, requirements such as safety, cost, and life are increasingly being raised.
A lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte solution, a separator, and an enclosure as main components. The positive electrode is composed of a positive electrode active material, a conductive material, a collector, and a binder (binding agent).
In general, a layered transition metal oxide typified by LiCoO2 is used as a positive electrode active material. However, the layered transition metal oxide is likely to release oxygen at a relatively low temperature of about 150° C. in a fully charged state and the thermal runaway reaction of a battery may possibly be caused by the release of oxygen. Thus, in the case of using a battery containing such a positive electrode active material in a portable electronic device, accidents such as the heat generation and ignition of the battery may possibly occur.
Therefore, olivine-structured iron lithium phosphate (LiFePO4) and derivatives thereof are expected because olivine-structured iron lithium phosphate has a stable structure, does not release oxygen under abnormal conditions, and is more inexpensive than LiCoO2. Japanese Unexamined Patent Application Publication No. 2008-166207 (Patent Literature 1) reports LiFe1-xZrxP1-ySiyO4 in which an Fe site and a P site are substituted with Zr and Si, respectively, as an iron lithium phosphate derivative.